plainness has 3 distinct synonyms spread across 2 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the state of being unmixed with other material
“the plainness of vanilla ice cream”
clarity as a consequence of being perspicuous
the appearance of being plain and unpretentious
an appearance that is not attractive or beautiful
“fine clothes could not conceal the girl's homeliness”
Synonyms, antonyms and the broader and narrower relations for plainness come from Princeton University’s WordNet. Full sources and licences.